There are large numbers of them in the wild; they are not currently (2011) threatened in the least. They can live up to 20 years.
Males are black and white; females are brown.
They dive for crustaceans and molluscs, particularly crabs and mussels. While sitting on their eggs, though, females will go without food for as much as three weeks. Groups of females often share the chores of raising their collective young.
The down from the breasts of females is used as eiderdown. The down can be harvested leaving the birds alive and unharmed — the birds themselves will pluck it from their own breasts to use it for their nests. This down is soft and warm, but light, and is considered very desirable for pillows and quilts.
In Iceland, the birds are protected from hunting year round, as there is a large, lucrative eiderdown industry around the birds.
The ducks of Eider eggs are good eating, but the meat can be tough and fishy tasting.
Eiderdown has now been replaced with cheaper alternatives: down from farm geese, and synthetics.